As used herein, the term “handoff” is intended to include transfers between networks of the same type and networks of different types, and also to include transfers while a mobile station is in idle mode and transfers while a mobile station is in an active communication session such as a telephone call.
A mobile station that is able to communicate with access points (APs) of wireless local area networks (WLANs) will typically make a decision to trigger a handoff based on the received signal strength indicator (RSSI) of signals received at the mobile station. The decision may be based on signals that originate from the AP with which the mobile station is currently associated or on signals that originate from other APs or on both types of signals. The other APs may belong to the same WLAN as the AP with which the mobile station is currently associated or may belong to a different WLAN. Handoffs from one WLAN to another may require more time to complete than handoffs within the same WLAN. For example, if the mobile station needs to obtain a new Internet Protocol (IP) address as part of the handoff, then the handoff will typically require more time than if the mobile station could maintain its current IP address. New IP addresses may be required if the mobile station performs the handoff across networks or across subnets of the same network.
If the mobile station is also able to communicate with base stations of a wireless wide area network (WWAN), for example, a cellular telephone network, then the decision to trigger a handoff from a WLAN to the cellular network is also typically made based on the RSSI of signals received at the mobile station. The decision may be based on signals that originate from the AP with which the mobile station is currently associated or on signals that originate from one or more of the base stations or on both types of signals. Handoffs from a WLAN to a cellular network typically require more time to complete than handoffs from one AP to another AP.
While many different schemes for triggering handoffs have been proposed, most schemes are directed at achieving one or more of the following general objectives:    a) A mobile station should look to perform a handoff before it loses its WLAN connection;    b) A handoff ought to take a minimum amount of time; and    c) A handoff from a WLAN to a cellular network ought to be completed while the mobile station is still in an area of overlapping coverage between the WLAN and the cellular network.
However, it is well known that under certain circumstances the handoffs fail to achieve these objectives. If a WLAN connection is lost or if its link quality deteriorates significantly before or during a handoff, communications over the WLAN connection may suffer. For example, a telephone call being carried over the WLAN connection may be disconnected or suffer unacceptable noise or delays.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.